This invention relates to a device for connecting and disconnecting the swivel of an oscillating marine platform, the said swivel being arranged between the lower end of the shaft supporting the bridge and the base fixed on to the sea bed.
French Patent Application No. 79 31842 filed in the name of the assignees describes a device permitting the assembly and dismantling of a swivel. The spherical convex surface is attached to the end of a conical pivot member which is located in a sealed casing in the end of the shaft. Flanges provided on the periphery of the casing and on the pivot member enable the pivot to be an integral part with the shaft. Deformable fittings are fixed by means of supports, on the one hand, on the convex surface and, on the other hand, on a bush which can cooperate with a concave spherical cap fixed in the base. Flanges fixed respectively on the cap and on the bush enable them to be integrated.
The weight of the unit comprising the shaft, bridge and ballast is made so as to be greater than Archimedes pressure and the upthrust due to the action of the elements. The swivel is therefore always under compression and the deformable fittings are not submitted to any tension which could cause them damage.
If it is desired to change the swivel, then the connections between the pivot member and the casing fixed in the shaft are removed, and the shaft is made lighter by emptying the floats which it carries, so as to free the pivot member. The shaft is then laterally displaced. Lifting means are attached to the pivot member and the connections between the bush and the cap fixed to the base are removed. The unit comprising the pivot member, deformable fitting and bush is reassembled for possible changing of the fittings.
This device with a pivot member and cap enables the swivel to be changed without having to dismantle the bridge installations before positioning the unit of the shaft and base.
Nevertheless, changing the swivel always takes a certain amount of time, during which the shaft and the bridge rising above it, disconnected from the base, are held solely by anchors on the sea bed. This phase becomes particularly delicate if there is a change in the condition of the sea due to bad weather, and this invention aims to eliminate this phase and to be at least partly independent of meteorological conditions. To achieve this, a double concentric swivel is provided, one of the swivels being used solely to make fast the shaft while the bearing swivel is being changed. This change is carried out by raising the bearing swivel along the axis of the shaft and through one part of the mooring swivel which is in the form of two hemispherical coaxial bushes of a type similar to that shown in U.S. Pat. No. 3,522,709.